430 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(6) Pettenkofer's Test for Bile Salts. A fragment of cane sugar 

 is dissolved in a little bile which has been diluted 10 times with 

 water ; when it has dissolved, about 5 c.c. of concentrated sulphuric 

 acid are run into the bottom of the test tube and shaken gently. A 

 purple colour develops slowly. Furfural is formed by the action of 

 the concentrated sulphuric acid on the sugar ; this reacts with the 

 bile acids, giving the purple colour. Excess of sugar must be avoided 

 as it may be charred by the strong acid and spoil the colour. The 

 colour disappears on diluting with water and is only stable in the 

 presence of strong sulphuric acid. If a portion of the purple liquid be 

 diluted with 50 per cent, sulphuric acid and examined in the spectro- 

 scope, two absorption bands, the one between C and D, nearer D, and 

 the other in the green, can be observed. 



This test is sometimes performed by shaking up the bile with a 

 little sugar solution so as to obtain a froth ; on pouring in the 

 concentrated sulphuric acid the colour appears where it has come in 

 contact with the froth. 



(7) Hay's Test for Bile Salts (Surface Tension Test). A little 

 bile in a test tube is diluted with water and some flowers of sulphur 

 are sprinkled on the surface. They sink. If the experiment be repeated 

 with pure water, the particles of sulphur will float. 



On performing the same test with strong mineral acids, ammonia, 

 brine, ammonium sulphate solution, etc., the sulphur floats. It sinks 

 in alcohol, ether, chloroform, olive oil, etc., in fact in all liquids with a 

 surface tension less than 60 dynes per sq. cm. 



This test depends upon the power of the bile salts to lower the 

 surface tension of water. It is particularly valuable for detecting bile 

 salts in urine where other coloured substances may interfere with 

 Pettenkofer's test. Alcohol, which has a low surface tension, must, if 

 present, be previously removed by evaporation. 



Grunbaum has described a method of estimating bile salts in urine which 

 depends on this property of bile salts. The rate of escape of. urine from 

 standard capillary tubes is measured ; the higher the concentration the greater 

 is the rate. 



Draughtsmen employ this property of bile salts in making tracings 

 on oiled paper on which the ink collects in drops and does not spread. 

 On treating the paper with ox bile and allowing it to dry the difficulty 

 is overcome owing to the reduction in surface tension. This experi- 

 ment with oiled paper treated with bile may be tried with advantage. 



In the same way oil will pass through a filter paper moistened with 

 dilute bile solution, whereas it will not pass through a paper moistened 

 with water. This statement may easily be verified. 



